GROUND SOURCE
EARTH COUPLING
DESIGN PRINCIPLES
Michael A. Weigand, P.E., Principal
Weigand Associates, Inc.,
Gaithersburg, MD
EARTH COUPLED WATER SOURCE
HEAT PUMP (WSHP) SYSTEMS
• WSHP extracts or rejects heat
from one medium (a source) to
another (a load).
– Loads
• Forced air for heating or cooling.
• Chilled or hot water for HVAC systems.
• Domestic hot water.
– Source
• The earth.
GROUND SOURCE HEAT
EXCHANGERS
Heat Source/Sink for Earth
Coupled Water Source Heat
Pump Systems
Heat is continually supplied to the ground in the form of solar energy. Approximately 46%
of the sun’s energy is absorbed by the earth. The remaining 54% is either reflected back
into space or absorbed by the earth’s atmosphere. At a depth of approximately 15 feet
the ground temperature remains fairly constant, with an average temperature between
42-77°F year-round, depending on the local climate, terrain and soil type.
Intent
• Extract Heat
• Reject Heat
• Provide economical system
operation via:
– Fluid temperature for ideal equipment
efficiency.
• This is the primary efficiency controlling
factor
– Energy balancing
HOT
Cool
Heat
Cool
Heat
Heat
Heat
COOL
COLD
Warm
Cold
Cold
Warm
rm
Wa
WARM
Wa
rm
rm
Wa
Common misconceptions about
the geothermal earth coupling
• Temperature: Water pumped out of the
ground will be hot (i.e. a geothermal hot
spring).
– Loop temperatures can range from 25 °F to
100 °F
• Geothermal earth coupling always has a
higher first cost.
– Earth coupling costs must be balanced
against savings from: less equipment, less
building area, lower utility connection fees.
Ground Source Heat
Exchangers
• Ground source loops can be installed in a
variety of ways, depending on:
– Needs
• Required heat transfer quantity and rate
– Opportunity
• Available area
• Geographic features
• Geological conditions
– Economics
• First costs
– Excavation costs
– Local market conditions
• Life cycle costs
Explanation of the methods:
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Vertical Closed Loop
Horizontal Closed Loop (Slinky)
Surface Water Loop
Open Loop
Vertical Closed Loop
Vertical Closed Loop
• Vertical Bore Holes 200 to 450 feet deep.
– Undisturbed ground temperature does not change at these
depths.
• Arrayed on a minimum of 15’x15’ grid
– The closer together the more heat build-up & higher risk bore
holes might hit each other.
– Recommend 20’x20’ when available.
– 25’x25’ is ideal but usually impractical
• Provides sufficient core volume to eliminate heat build-up in ground.
– Greater the perimeter to area ratio the higher the efficiency.
• Average 250 to 300 s.f. of surface area per ton.
• Average 180 – 250 ft of borehole per ton.
Vertical Closed Loop
Vertical Closed Loop
Vertical Closed Loop
Vertical Closed Loop
Vertical Closed Loop
Vertical Closed Loop
Vertical Closed Loop
Horizontal Closed Loop (Slinky)
Horizontal Closed Loop (Slinky)
• Horizontal loops installed in trenches 5+ feet below
ground surface.
– Deeper trenches would require expensive shoring.
– Trenches should be a minimum of 15 ft apart.
• Undisturbed ground temperatures may change
seasonally depending upon geographic location.
• Average 2,500 s.f. of surface area per ton.
– More applicable for smaller projects
– Or projects with large available land area
• Trench lengths average 150 – 220 ft per ton.
• Installation under paved areas is not recommended.
Horizontal Closed Loop (Slinky)
Shallow Ground Temperature
Variation with Season
Surface Water Loop
Surface Water Loop
• Coils installed in a body of water.
– Lake, pond, river.
– Body of water may be constructed on-site.
– Near the building
Surface Water Loop
• Coils installed in a body of water.
– Lake, pond, river.
– Body of water may be constructed on-site.
– Near the building
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Mass and surface area of water is critical.
Pond depths are usually 12 feet minimum.
Pond sizes average 10 – 50 tons per acre.
May need regulatory approval.
Potential temperature impact on aquatic life.
Surface Water Piping Design
• Typically 350 ft coils of ¾” HDPE pipe.
• Each coil rejects ~ 1 ton.
• Coils are assembled in frames – floated into
place – then filled with water and allowed to sink.
• Antifreeze is common even in warm climates.
• Special attention is required where piping leaves
the water body to prevent damage.
Summer heat transfer occurs at the water surface via
evaporation, so the process closely tracks water
temperature and ambient wet bulb temp.
In winter, when the pond could be frozen, heat transfer
is dominated by contact between the loops, the bottom
water and the soil surface at the bottom of the pond.
Ideal Temperature vs. Pond Depth
Open Loop
Open Loop
• Directly uses ground water for heat exchange.
• Aquifers that can furnish water at high flow rates
are generally of coarse material such as gravel,
but not clay, sand or bedrock
• Key benefit is a constant water temperature
(50°F to 60°F) at an ideal temperature.
• An open loop earth coupling can be the lowest
first cost and the highest efficiency method.
Open Loop
• These have the highest maintenance
costs.
– System fouling from untreated water.
– Potential for clogging intake screens.
Open Loop
• Two types:
– Reinjection (Diffusion) Wells
– Surface Discharge
Reinjection
Reinjection
• Commonly used in coastal areas where
geological characteristics allow reinjection
wells to return the water back to the
aquifer.
• Source well draws water from earth.
• Reinjection (diffusion) well returns the
water.
• Separation in feet = (btu/hr design X
0.2)0.5
• Must be approved by the U.S. EPA and
local AHJ’s.
Surface Discharge
• Usually small scale applications only.
• Must be able to contain the discharge
volume.
• Need an adequate source
Open Loop Design Issues
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Untreated water must be potable (no brackish or rotten egg odors).
The water table should usually be within 100 feet of the surface.
The volume of available water should be equal to twice the peak
demand flow.
Wells drilled into shallow bedrock and specifically into karst
(cavernous limestone) formations function best if the casing can
extend below the pumping water level. (Shallow water is easily
contaminated.)
Wells drilled into sand and gravel formations function best if a well
screen is installed. Well screens dramatically increase the capacity
of a well and assure a longer lasting and more reliable water supply.
Screens for sand have smaller openings than for gravel.
Screens for diffusion wells must be twice the size of screens for
supply wells
Common Design Considerations
• Well depths and costs
– Sand and gravel
• wells are drilled with an air rotary drilling rig. The
steel casing is driven down with a casing hammer
as the well is being drilled.
• These are usually shallower and less expensive
per foot.
• 6”, 8”, & 12” diameter boreholes.
– Bedrock
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Wells are drilled with a down-hole hammer.
Have casing in the upper part of the well.
Longer time to drill, more expensive.
Deeper holes
6” & 8” diameter boreholes.
Common Design Considerations
• Fluid vs. Ground Temperature
Difference
– provides the impetus for the energy to move
Common Design Considerations
• Fluid vs. Ground Temperature
Difference
– Deep Earth Temperature
Common Design Considerations
• Fluid vs. Ground Temperature
Difference
– Temperatures near surface
Common Design Considerations
• Fluid vs. Ground Temperature
Difference
– Design fluid temperature
• Balancing ground exchanger size with
equipment performance
• Typical SWT for cooling = ground temp + 30 F
• Typical SWT for heating = ground temp – 10 F
• Need to add propylene glycol if return
temperatures approach 32 F.
Common Design Considerations
• Thermal conductivity/resistance
– Pipe
Common Design Considerations
• Thermal conductivity/resistance
– Pipe
– Grout (vertical wells)
Common Design Considerations
• Thermal conductivity/resistance
– Pipe
– Grout (vertical wells)
– Ground
Common Design Considerations
• Thermal conductivity/resistance
– Pipe
– Grout (vertical wells)
– Ground
– Best to determine with an In-Situ
Thermal Conductivity Test
Common Design Considerations
• Effects of ground water
– Ground water movement through the bore
hole field can have a large impact on its
performance.
– Ground water recharge (vertical flow) and
ground water movement (horizontal flow) can
all carry away large amounts of energy.
– Evaporation can also cool the surface soil and
improve horizontal loop performance
Common Design Considerations
• Energy load and heat balance
over time
– Ground temperatures change as a result of
system heat transfer:
• Over the short term
• Annually
• Long Term
– A commercial well field may have a 6 degree
F increase every 10 years.
– May need to add additional well field area over
time
Common Design Considerations
• Loop flow rates
– Flow rate should be based upon connected
equipment load
– ASHRAE 90.1 requires 10 HP pump systems
to be variable flow.
– The design load flow rate will be less than max
pumping rate.
– Laminar flow concerns
• Laminar flow is acceptable at part load conditions
since the plastic pipe thermal resistance is dominate.
• Turbulent flow is required for design load conditions.
Common Design Considerations
• Loop flow rates
– Flow rate should be based upon connected
equipment load
– ASHRAE 90.1 requires 10 HP pump systems
to be variable flow.
– The design load flow rate will be less than max
pumping rate.
– Laminar flow concerns
Common Design Considerations
• Piping
– High Density Polyethylene Piping (HDPE) with thermal
fused joints
– HDPE piping uses Standard Dimension Ratio (SDR) not
traditional schedule sizes (SoDR & SiDR).
– Thermally fused piping must be SODR which is based
on outside diameter
– SDR pressure ratings are consistent regardless of pipe
diameter.
• SDR-17 is generally rated at 100 PSI
• SDR-11 is generally rated at 160 PSI
• SDR-9 is generally rated at 200 PSI
– Pipe selection needs to consider:
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Elevation head
System fill pressure
½ of pump head
External Pressures (from water table – a benefit)
Positive & Negative Aspects
Comparison Summary
• Vertical Loop
– POSITIVES
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Smallest area requirement
No surface temperature impacts
Closed Loop – No ground water quality issues
Potential for mass energy storage (balancing)
– NEGATIVES
• Potential for field temperature rise over time
• May need to add wells over time
• High drilling cost
Positive & Negative Aspects
Comparison Summary
• Horizontal Loop
– POSITIVES
• Closed Loop – No ground water quality issues
• Can be less expensive than drilling
• Does not require special equipment
– NEGATIVES
• Surface temperature impact (seasonal)
• Requires large surface area
Positive & Negative Aspects
Comparison Summary
• Surface Water
– POSITIVES
• Closed Loop – No ground water quality issues
• Summer evaporation effect
• Can be lowest first cost closed loop system
– NEGATIVES
• Surface temperature impact (seasonal)
• Requires a large water body
• Antifreeze: derating of equipment, increased viscosity
Positive & Negative Aspects
Comparison Summary
• Open Loop
– POSITIVES
• Ideal source fluid temperature
• No temperature loading issues
• Can have the highest system efficiency
– NEGATIVES
• Open Loop – Water quality issues
• May need a secondary heat exchanger
• Potential for additional equipment maintenance issues
• Well screen maintenance
• Regulatory compliance
• Requires a high flow/volume capacity source
Credits
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ASHRAE
– Ground Source Heat Pump Systems: Design of Geothermal
Systems for Commercial and Institutional Buildings
McQuay
– http://www.mcquay.com/McQuay/DesignSolutions/Geothermal
NYC Department of Design and Construction
– Geothermal Heat Pump Manual
Questions?